Improving the efficiency of extracting light from a light-emitting device, such as a light-emitting diode (LED), primarily centers on two areas: (1) increasing the efficiency of the semiconductor material in generating light per unit of drive current (i.e. photons out per electrons in) and (2) improving the manner in which generated light is extracted from the light-emitting device. The present invention addresses the latter by providing a method for improving light extraction from a light-emitting device, such as an LED that is on a suitable substrate, such as a wafer used in the production of semiconductor devices. Light-emitting devices, such as an LED, suffer a large light loss because most of the light is guided through the active layer and lost out of the sides of the semiconductor material. Innovative designs in the chip or the packaging of the LED must then be employed to gather as much of the light as possible. Some of these collection mechanisms utilize parabolic cups in which the LED is packaged. Such designs are manufacturing-intensive and normally result in a low quality of the gathered optical light beam. The present invention allows for direct integration of the light extraction device into a standard semiconductor manufacturing process and does not involve the deposition of metal directly onto the structure. Thus, the subject invention is conducive to high volume and high yield manufacturing at significantly lower cost than previous processes for providing light extraction devices. The present invention also allows smaller packaging, which permits miniaturization of the final product and much more flexibility in the lighting applications for which such devices may be utilized.
Previous on-chip light extraction solutions include surface texturing (see U.S. Pat. No. 5,779,924), a thick epitaxial semiconductor layer (see U.S. Pat. No. 6,133,589), finely spaced reflective electrodes (see U.S. Pat. No. 6,258,618), and chip shaping (U.S. Pat. No. 6,323,063). These solutions are significantly different than the present invention in that they either attempt to extract the light more efficiently by distributing the current across the chip or they roughen the surface in order to increase the extraction efficiency. The subject invention comprises the fabrication of a micro-optical structure directly on the chip containing the light source, to direct a higher percentage of the light in a desired direction.
The claimed invention provides the following advantages over prior light extracting technologies: (1) it allows for efficient light collection and an optical light beam that is confined to an emission area slightly smaller than that of the LED chip, (2) the process can be directly integrated into a current semiconductor manufacturing process, and (3) it eliminates the need for any complicated packaging process, which usually requires the use of mirrored surfaces, cups or other packages. Another benefit of the subject invention is that it allows the addition of a microlens on top of the light emitting structure, once it is formed. The present invention preferably uses a gray-scale process to fabricate a mirrored surface, which is controllable, in order to form a simple flat mirror or a shaped surface to thereby provide a lensing effect to the mirror.